Methods for locating formation fractures and monitoring well completion using streaming potential transients information

ABSTRACT

Earth formations are characterized by using an array of electrodes which can measure streaming potentials in the formation, and by interpreting the data obtained by the electrodes. The electrodes are placed on a wireline tool, a LWD tool, or in a fixed manner about a completed wellbore. The measured streaming potentials are generated by drilling with an overbearing pressure, slitting the mudcake in a borehole, acid injection, or any of various other manners which causes fluid movement. The data obtained is interpreted to locate fractures, measure formation permeability, estimate formation pressure, monitor drilling fluid loss, detect abnormal pressure, etc. Particularly, a streaming potential voltage transient having a double peak profile signifies the presence of a formation fracture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to commonly-assigned U.S. patentapplication Ser. No. ______, entitled “WHILE-DRILLING APPARATUS FORMEASURING STREAMING POTENTIALS AND DETERMINING EARTH FORMATIONCHARACTERISTICS”; commonly-assigned U.S. patent application Ser. No.______, entitled “WIRELINE APPARATUS FOR MEASURING STREAMING POTENTIALSAND DETERMINING EARTH FORMATION CHARACTERISTICS”; and commonly-assignedU.S. patent application Ser. No. ______, entitled “COMPLETION APPARATUSFOR MEASURING STREAMING POTENTIALS AND DETERMINING EARTH FORMATIONCHARACTERISTICS”; each such U.S. patent application being simultaneouslyfiled herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to the hydrocarbon industry. Moreparticularly, this invention relates to apparatus and methods formeasuring streaming potentials resulting from pressure transients in anearth formation traversed by a borehole. This invention also relates tomanners of making determinations regarding earth formationcharacteristics as a result of streaming potential measurements. Onesuch characteristic is the permeability of the formation at differentdepths thereof, although the invention is not limited thereto.

2. State of the Art

The history with respect to the possibility of making streamingpotential measurements in a downhole formation is a long one. In U.S.Pat. No. 2,433,746, (1947) Doll suggested that vigorous vibration of adownhole apparatus in a borehole could generate pressure oscillationsand fluid movement relative to the formation which in turn could giverise to measureable streaming potentials due to an electrokineticpotential phenomenon. In U.S. Pat. No. 2,814,017, (1957) Doll suggestedmethods for investigating the permeabilities of earth formations byobserving the differences in phase between periodic pressure wavespassed through the formations and potentials generated by theoscillatory motion of the formation fluid caused by these pressurewaves. Conversely, a periodically varying electric current was suggestedto be used to generate oscillatory motion of the formation fluid, whichin turn generated periodic pressure waves in the formation. Measurementswere to be made of the phase displacement between the generating and thegenerated quantities and a direct indication of the relativepermeability of the formation thereby obtained.

In U.S. Pat. No. 3,599,085, to A. Semmelink, entitled, “Apparatus ForWell Logging By Measuring And Comparing Potentials Caused By SonicExcitation”, (1971) the application of low-frequency sonic energy to aformation surface was proposed so as to create large electrokinetic, orstreaming, pulses in the immediate area of the sonic generator. Inaccordance with the disclosure of that patent, the electrokinetic pulsesresult from the squeezing (i.e. the competition of viscosity andinertia) of the formation, and the streaming potential pulses generateperiodic movements of the formation fluid relative to the formationrock. The fluid movement produces detectable electrokinetic potentialsof the same frequency as the applied sonic energy and having magnitudesat any given location directly proportional to the velocity of the fluidmotion at that location and inversely proportional to the square of thedistance from the locus of the streaming potential pulse. Since thefluid velocity was found to fall off from its initial value withincreasing length of travel through the formation at a rate dependent inpart upon the permeability of the formation rock, it was suggested thatthe magnitude of the electrokinetic potential at any given distance fromthe pulse provided a relative indication of formation permeability. Byproviding a ratio of the electrokinetic potential magnitudes (sinusoidalamplitudes) at spaced locations from the sonic generator, from whichelectrokinetic skin depth may be derived, actual permeability can inturn be determined.

In U.S. Pat. No. 4,427,944, (1984) Chandler suggested a stationary-typeborehole tool and method for determining formation permeability. Theborehole tool includes a pad device which is forced into engagement withthe surface of the formation at a desired location, and which includesmeans for injecting fluid into the formation and electrodes formeasuring electrokinetic streaming potential transients and responsetimes resulting from the injection of the fluid. The fluid injection iseffectively a pressure pulse excitation of the formation which causes atransient flow to occur in the formation. Chandler suggests ameasurement of the characteristic response time of the transientstreaming potentials generated in the formation by such flow in order toderive accurate information relating to formation permeability.

In U.S. Pat. No. 5,503,001 (1996), Wong proposed a process and apparatusfor measuring at finite frequency the streaming potential andelectro-osmotic induced voltage due to applied finite frequency pressureoscillations and alternating current. The suggested apparatus includesan electromechanical transducer which generates differential pressureoscillations between two points at a finite frequency and a plurality ofelectrodes which detect the pressure differential and streamingpotential signal between the same two points near the source of thepressure application and at the same frequency using a lock-in amplifieror a digital frequency response analyzer. According to Wong, because theapparatus of the invention measures the differential pressure in theporous media between two points at finite frequencies close to thesource of applied pressure (or current), it greatly reduces the effectof background caused by the hydrostatic pressure due to the depth of theformation being measured.

Despite the long history and multiple teachings of the prior art, it isbelieved that in fact, prior to field measurements made in support ofinstant invention, no downhole measurements of streaming potentialtransients in actual oil fields have ever been made. The reasons for thelack of actual implementation of the proposed prior art embodiments areseveral. According to Wong, neither the streaming potential nor theelectro-osmotic measurement alone is a reliable indication of formationpermeability, especially in formations of low permeability. Wong statesthat attempts to measure the streaming potential signal with electrodesat distances greater than one wavelength from each other are flawedsince pressure oscillation propagates as a sound wave and the pressuredifference would depend on both the magnitude and the phase of the wave,and the streaming potential signal would be very much lower sinceconsiderable energy is lost to viscous dissipation over such a distance.In addition, Wong states that application of a DC flow to a formationand measurement of the response voltage in the time domain will not workin low permeability formations since the longer response time and verylow streaming potential signal is dominated by drifts of the electrodes'interfacial voltage over time. Thus, despite the theoreticalpossibilities posed by the prior art, the conventional wisdom of thoseskilled in the art (of which Wong's comments are indicative) is thatuseful streaming potential measurements are not available due to lowsignal levels, high noise levels, poor spatial resolution, and poorlong-term stability. Indeed, it is difficult to obtain pressuretransient data with high spatial resolution as the borehole isessentially an isobaric region. The pressure sensor placed inside theborehole cannot give detailed information on the pressure transientsinside the formation if the formation is heterogeneous. To do so, it isnecessary to segment the borehole into hydraulically isolated zones, adifficult and expensive task to perform. Further, it will be appreciatedthat some of the proposed tools of the prior art, even if they were tofunction as proposed, are extremely limited in application. For example,the Chandler device will work only in drilled boreholes prior to casingand requires that the tool be stationed for a period of time at eachlocation where measurements are to be made. Thus, the Chandler devicecannot be used as an MWD/LWD (measurement or logging while drilling)device, is not applicable to finished wells for making measurementsduring production, and cannot even be used on a moving string of loggingdevices.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide methods andapparatus for measuring streaming potential in an earth formation.

It is another object of the invention to provide methods and apparatusfor measuring streaming potentials in a formation while drilling aborehole.

It is a further object of the invention to provide methods and apparatusfor measuring streaming potentials in a formation around a devicepermanently installed in a wellbore.

It is also an object of the invention to provide methods and apparatusfor measuring streaming potentials in a formation with a moving boreholetool.

It is an additional object of the invention to provide methods ofdetermining formation characteristics using streaming potentialsmeasurements.

Another object of the invention is to provide methods of characterizingfractures in a formation using streaming potential measurements.

A further object of the invention is to provide methods of determiningone or more of formation permeability, skin permeability, effectivefracture permeability, and horizontal and vertical permeabilities of aformation using streaming potential measurements.

In accord with these objects, which will be discussed in detail below,different methods and apparatus for measuring streaming potential in anearth formation are provided. A first embodiment of the inventionrelates to measuring streaming potential while drilling a borehole. Forpurposes herein, measurement-while-drilling (MWD) andlogging-while-drilling (LWD) applications will be consideredinterchangeable. A second embodiment of the invention relates tomeasuring streaming potential with a borehole tool which is adapted tomake measurements while moving through the borehole. A third embodimentof the invention relates to measuring streaming potential with apparatuswhich is permanently installed (e.g., cemented) about the wellbore. Allembodiments of the invention can be utilized to find characteristics ofthe formation. In particular, since the streaming potential measurementrelates directly to fluid flow, the streaming potential measurements canbe used to track flow of fluids in the formation. In turn, thisinformation may be used to find the permeability of the formation indifferent strata about the borehole and/or to find and characterizefractures in the formation.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a completed horizontal well havingelectrodes deployed thereabout for purposes of measuring streamingpotentials.

FIG. 2 is a schematic diagram of electrodes mounted on insulated jointsections of the sand-screen completion of FIG. 1.

FIG. 3 is a plot of pressure transients measured for two of the zonesshown in FIG. 1.

FIG. 4 is a plot showing pressure transients and streaming potentialsover time for the well of FIG. 1.

FIG. 5 is a plot showing the streaming potentials measured by electrodesin zone 2 of FIG. 1.

FIG. 6 is a plot showing the streaming potentials measured by electrodesin zone 3 of FIG. 1.

FIG. 7 is a plot showing voltage drifts of the electrodes in zone 1 ofFIG. 1.

FIG. 8 is a plot showing streaming potentials measured by electrodes inzone 1 of FIG. 1.

FIG. 9 is a schematic diagram of the well of FIG. 1 showing qualitativedeterminations made from information obtained by the electrodes disposedabout the well.

FIG. 9 a is a schematic representing a forward model of a heterogeneousformation.

FIG. 9 b is a plot of streaming potentials generated by the forwardmodel of FIG. 9 a.

FIG. 9 c is a schematic representing a forward model of a fracturedformation.

FIG. 9 d is a plot of streaming potentials generated by the forwardmodel of FIG. 9 c.

FIG. 10 is a schematic diagram of a completed vertical well havingelectrodes deployed thereabout for purposes of measuring streamingpotentials.

FIG. 11 is a schematic diagram of the manner in which electrodes weremounted in the completed well of FIG. 10.

FIG. 12 is a plot of the uphole pressure applied to the well of FIG. 10over a period of days.

FIG. 13 is a plot showing the uphole pressure of FIG. 12 and thestreaming potentials measured by a series of electrodes in a reservoirlocation shown in FIG. 10.

FIG. 14 is an enlarged version of a portion of FIG. 13.

FIG. 15 is a plot showing the uphole pressure of FIG. 12 and thestreaming potentials measured by a group of electrodes above thereservoir location.

FIG. 16 is an enlarged version of a portion of FIG. 15.

FIG. 17 is a plot showing the uphole pressure of FIG. 12 and thestreaming potentials measured by a group of electrodes below thereservoir location.

FIG. 18 is a schematic diagram of the well of FIG. 10 showingqualitative determinations made from information obtained by theelectrodes disposed about the well.

FIG. 18 a is a schematic representing a forward model of a verticalproducing well having a fracture.

FIG. 18 b is a plot of streaming potentials generated by the forwardmodel of FIG. 18 a.

FIG. 19 is an enlarged version of a portion of FIG. 17 which is used toshow the stability of the electrodes.

FIG. 20 is a schematic diagram of an open hole completion withelectrodes located about an insulated zone surrounding a tubing.

FIG. 21 is a schematic diagram of a cased-hole completion withelectrodes incorporated into the casing.

FIG. 22 is a schematic diagram of an LWD tool with streaming potentialelectrodes disposed thereon.

FIG. 23 is a schematic diagram of a wireline tool having streamingpotential electrodes disposed thereon.

FIG. 23 a is a schematic representing a forward model of a wireline toolwhich is adapted to slit borehole mudcake.

FIG. 23 b is a plot of streaming potentials generated by the forwardmodel of FIG. 23 a with respect to an uninvaded zone.

FIG. 23 c is a plot of streaming potentials generated by the forwardmodel of FIG. 23 a with respect to an invaded zone.

FIG. 23 d is a plot generated by the forward model of FIG. 23 a of thesensitivity of the streaming potential with respect to depth ofinvasion.

FIG. 23 e is a plot which shows an inversion for permeability ofsynthetic data and a best fit for a five parameter model.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to turning to the Figures, some theoretical considerationsgoverning the physics of the invention are useful. In reservoir rocksthere exists a thin charged double layer at the interface between therock matrix and the water in the pore. The matrix surface is negativelycharged, and the water is positively charged. When water moves under apressure gradient ∇p, an electrical current i_(e) is created with thewater current. The electrical current is proportional to the watercurrent, which is proportional to the pressure gradient:i_(e)=L ∇p   (1)where L is a coupling constant which is a property of the rock.

Pressure transients are created in the formation by many differentoperations that occur over the lifetime of a well such as drilling, mudinvasion, cementing, water and acid injection, fracturing, and oil andgas production. Pressure transient testing is an established techniqueto determine reservoir properties such as permeability, reservoir size,and communication between different zones and between different wells.As is set forth below, streaming potential transients associated withthe pressure transients can also be used to determine these properties.

The modeling of the reservoir pressure p can be carried out withmultiphase flow models. For the modeling of the streaming potential, itis useful to start with the diffusion equation of a single-phase flow:$\begin{matrix}{{{{\nabla{\cdot \frac{k}{\mu}}}{\nabla p}} = {\phi\quad c\quad\frac{\partial}{\partial t}p}},} & (2)\end{matrix}$where k is the permeability, μ is the viscosity, φ is the porosity, andc is the fluid compressibility. From the modeled pressure field p, thestreaming potential V can be calculated by solving the Poisson Equation:−∇·ρ∇V=∇·L∇p,   (3)where ρ is the electrical conductivity.

From Eq. (2) it follows that the time Δt for a pressure transient andthe associated streaming potential transient created at the boreholesurface to diffuse through a distance Δx into the formation is given by$\begin{matrix}{{\Delta\quad t} \sim {\frac{\phi\quad c\quad\mu}{k}{\left( {\Delta\quad x} \right)^{2}.}}} & (4)\end{matrix}$The early time pressure and streaming potential transients are sensitivemainly to reservoir properties near the borehole, and the late timetransients are sensitive to reservoir properties both near the boreholeand farther away from the borehole. By interpreting the measuredtransients in a time ordered fashion, reservoir properties at differentdistances to the borehole can be determined. The interpretation ofpressure transients in this time ordered fashion is an established art.For example, early time pressure transients are used to determine damageto permeabilities or “skin”, and late time pressure transients are usedto determine reservoir boundaries.

The applications are much more limited if the steady state values of thestreaming potentials are the only measurements available. At a steadystate, equation (2) becomes $\begin{matrix}{{{\nabla{\cdot \frac{k}{\mu}}}\quad{\nabla p}} = 0.} & (5)\end{matrix}$The pressure drop Δp across a depth interval Δx is then proportional to$\begin{matrix}{{\Delta\quad p} \propto {\frac{\mu}{k}\quad\Delta\quad{x.}}} & (6)\end{matrix}$The drop in the streaming potential ΔV is related to Δp by$\begin{matrix}{{\Delta\quad V} = {{- \frac{L}{\sigma}}\quad\Delta\quad p}} & (7)\end{matrix}$which is proportional to $\begin{matrix}{{\Delta\quad V} \propto {\frac{L\quad\mu}{\sigma\quad k}\Delta\quad{x.}}} & (8)\end{matrix}$The steady state streaming potential can only give information on theaverage value of a reservoir property and as a result is dominated byintervals with high values of (Lμ)/(ρk). It is believed that in thepresence of a mudcake, the steady state streaming potential is dominatedby the mudcake and is insensitive to reservoir properties. Thepermeability of the mudcake is extremely low, and the steady statepressure drop mainly exists across the mudcake.

While in principle it is possible to determine reservoir properties atall distances to the borehole (i.e., radially from the borehole) byinterpreting the transients in a time ordered fashion, the criticalquestion in practice is whether the measurements can be made withsufficient quality: accuracy, spatial resolution, and stability overlong time. It is difficult to get pressure transient data with highspatial resolution as the borehole is essentially an isobaric region. Apressure sensor placed inside the borehole cannot give detailedinformation on the pressure transients inside the formation if theformation is heterogeneous. To do so, it would be necessary to segmentthe borehole into hydraulically isolated zones, a difficult andexpensive task to perform. On the other hand, the borehole is not anequipotential surface for electric current flow. Thus, streamingpotential transients may be measured by an array of electrodes placedinside the borehole and electrically isolated (i.e., insulated) one fromthe other and can provide equivalent information to that ofhydraulically isolated zone pressure transient testing because thestreaming potential is determined by the pressure gradient. In fact, byutilizing an array of isolated streaming potential electrodes, thestreaming potential can be measured with a higher spatial resolutionthan hydraulically isolated zone pressure transient testing.

Given the theoretical understandings above, according to one aspect ofthe invention, insulated electrodes are deployed in or about a boreholeor a well in order to measure streaming potential transients. Accordingto different embodiments of the invention, and as will be discussed inmore detail below, the electrodes may be deployed on insulated sectionsof a drill pipe in while-drilling (MWD or LWD) applications, or on thebody of a tool which is moved through the borehole in wireline loggingapplications. In post-completion applications, the electrodes may bedeployed on an insulating sonde placed in an open hole for an open-holecompletion, or on (or as part of) centralizers in sand-screencompletions, or in insulation surrounding a casing in a cementedcompletion. In a cased-hole completion with electrically isolated casingsections, the metal casings can serve as electrodes. Regardless of howthe electrodes are deployed, DC voltage differences indicative ofstreaming potentials are measured between a reference electrode andother electrodes of an array. Initial voltage difference values betweenthe reference electrode and other electrodes typically due to surfacechemistry differences of the electrodes are subtracted from all datasubsequent to the creation of pressure transients.

According to another aspect of the invention, the streaming potentialtransients are generated in any of various manners. According to oneembodiment of the invention associated with drilling a borehole, thepressure difference between the formation and the borehole creates mudinvasion, pressure transients and streaming potential transients. Inanother embodiment of the invention associated with wireline logging ofthe borehole, streaming potential transients are generated by providingthe wireline tool with one or more cutting edges mounted on one or moreretractable arms which cut slits across the mudcake while logging.Because of a large overbalancing pressure difference between theformation and the borehole, when the mudcake is slit, fluid will flowthrough the slit and the resulting pressure transient can be measured.According to another embodiment of the invention associated withcompletion and post-completion applications, streaming potentialtransients are generated by injection of completion fluid, cement,gravel, acids, fracturing propellant, water injection testing,production testing, etc. In fact, any change in the rate of productionwill also create streaming potential transients. As long as there is aflow of conducting fluids associated with pressure transients, astreaming potential transient will be created and will be measurablewith high precision using the deployed electrodes.

According to another aspect of the invention, data related to streamingpotential transients obtained by the electrodes is interpreted toprovide useful information. Those skilled in the art will appreciatethat the interpretation of pressure transient data (as opposed tostreaming potential transient data) to obtain reservoir properties suchas permeability is a well-established art. In formations with highpermeability, the pressure transients change with time rapidly, while informations with low permeability, the pressure transients change slowly.The streaming potential transients produced by the pressure transientsdepend on the formation permeability in the same way as the pressuretransients.

As will be appreciated by those skilled in the art, there existanalytical and numerical tools to model the pressure transients. Thereservoir parameters of interest can be determined by varying theparameters in the model until the calculated pressure matches with themeasured data. Formally, let R denote the set of reservoir parameters tobe determined, and let f_(p)(R)denote the modeled pressure transient. Amismatch between the modeled and the measured pressure transient isdefined:E _(p)(R)=∥f _(p)(R)−p∥  (9)The mismatch is minimized at R=R₀ to get the inverted values of thereservoir parameters.

The quantitative interpretation of streaming potential data to determinereservoir parameters such as formation permeability can be carried outin the same way as the interpretation of the pressure transient data.Let s denote a set of measured transients. The set may contain only thestreaming potential transients V, or it may contain both the streamingpotential transients and the pressure transients. Let f_(s)(R,L)denotethe modeled transients, which depend on an additional set of parameters:the coupling constants L in equation (3). (The conductivity σ inequation (3) is usually known from resistivity logging data.) A mismatchbetween the modeled and the measured pressure transients is definedaccording to:E _(s)(R,L)=∥f _(s)(R,L)−s∥.   (10)The mismatch is minimized at R=R₀ and L=L₀ to get the inverted values ofthe reservoir parameters.

It will be appreciated by those skilled in the art that the Poissonequation (3) is linear in the coupling constants L, since the couplingof the streaming potential back into the governing equations for thepressure by electro-osmosis is completely negligible. Therefore, theinversion for the coupling constants is a straightforward linearinversion. Indeed, the minimization of equation (10) is carried out intwo steps. The first step is to fix R and vary L, and find thesub-optimal minimum of the mismatch by solving a linear problem for L.The solution gives L as a function of R. The sub-optimal minimum is thena function of R only:E_(s1)(R)≡E_(s)(R,L(R)).   (11)The second step is a nonlinear search for the minimum of equation (11),containing the same number of unknowns as equation (9). Therefore, theadditional task of estimating the coupling constants does not add to thecomputational complexity or mathematical uncertainly to the inversionproblem.

According to another aspect of the invention, the measured streamingpotential transients associated with the fluid movement in the formationcan be used inter alia to: track movement of cement slurries duringcementing thereby detecting possible cementing problems; track slurriescarrying gravel thereby monitoring gravel packing; track acid movementduring injection of acid into the formation as acid injection willcreate streaming potential transients; monitor fracturing of formationsin real time; evaluate fracture jobs quantitatively; track watermovement resulting from water injection; improve the effectiveness ofpressure transient testing; and monitor reservoir parameter changes overlong periods of time, including water saturation, relative permeabilityand water cut.

Using the various aspects of the invention previously described, fieldtests were run on a horizontal production well, part of which is shownschematically in FIG. 1. The horizontal production well 100 of FIG. 1was completed in formation 105 with sand screens 114 (see FIG. 2) andsegmented into three zones with external casing packers 111 a, 111 b,111 c. The zone closest to the heel of the horizontal well is labeled asZone 1, the middle zone as Zone 2, and the zone closest to the toe asZone 3. Each zone was provided with a valve unit 113 a, 113 b, 113 crespectively, extending through the screen 114, with two pressuresensors 115-1 and 115-2 associated with each valve unit 113 (see FIG.2). Electrodes 118 were deployed as discussed below.

Turning now to FIG. 2, deployment of the electrodes 118 according to theinvention is seen. As seen in FIG. 2, the well 100 is completed withsand-screen sections 114 which are coupled together by insulated jointsections 116 to form the completion string. It will be appreciated thatthe screen sections cannot be electrically insulated from the formation105 or from the annulus fluid (not shown). The joint sections 116 areelectrically insulated. Mounted in the middle of each joint section is acentralizer 118. Because of the weight of the completion string 114,116,the centralizers 118 are always in good contact with the formation 105.Therefore, in accord with the invention, the centralizers 118 areequipped as electrodes with preferably high impedance voltagemeasurement circuits and are coupled to surface electronics by cablewires (not shown). Appropriate centralizer hardware is described in PCTApplication WO 02/053871.

It will be appreciated by those skilled in the art that the completionstring, being made of metal, forms a short circuit for electricalcurrents. The screen sections 114 used to complete well 100 were fifteenfeet long, and the joint sections 116 were five feet long. Since theinsulated joint sections 116 of the completion string covered only asmall area near the electrodes 118, much of the electrical currents wereable to leak through the exposed screen sections 114, resulting in thereduction of signal level. However, as shown hereinafter, there stillexisted significant levels of signal to be measured. It should be notedthat for quantitative interpretation, it is sufficient to include thecurrent leakage in the forward modeling.

As shown in FIG. 1, seven electrodes were provided per zone for a totalof twenty-one electrodes (labeled 118-1, 118-2 . . . , 118-21. With afifteen foot screen section and a five foot joint section, the distancebetween neighboring electrodes in the same zone was approximately twentyfeet. The distance between the nearest two electrodes in different zoneswas just over one hundred feet.

Pressure testing data gathered by the pressure gauges on both sides ofthe completion string showed that Zone 1 is hydraulically isolated fromZone 2 and Zone 3. This is seen in FIG. 3, since Zone 1 pressure 125 ais significantly higher than Zone 2 and Zone 3 pressures 125 b, 125 cthereby indicating isolation. Therefore, for the voltages of theelectrodes in Zone 1 (118-1 through 118-7), the reference electrode waschosen to be in Zone 2 or Zone 3, and for the voltages of the electrodesin Zone 2 and Zone 3, the reference electrode was chosen to be in Zone1.

In order to create a streaming potential transient, the three electricalvalves 113 a, 113 b, 113 c, and a rod pump (not shown) at the formationsurface were utilized to control the fluid flow. The fluid in theannulus of each zone flowed into the tubing through the valve opening.The pressure gauge 115-1 on the tubing side of the opening measured thetubing pressure, and the pressure gauge 115-2 on the annulus sidemeasured the pressure in the annulus region between the formation andthe screen. By turning the pump on and off and by opening and closingthe valves 113, pressure transients were created in the formation 105and measured by the pressure gauges 115-2 on the annulus side.

For each of the three zones, the annulus pressure was equal to theformation pressure. As seen in FIG. 3, the Zone 2 and Zone 3 annuluspressures 125 b, 125 c were approximately equal, indicating that the twozones are in hydraulic communication. The Zone 1 annulus pressure 125 awas higher, indicating that Zone 1 is hydraulically isolated. By pumpingthe fluid out of the borehole, the tubing pressure was kept at a value˜150 psi lower than the annulus pressures in Zone 2 and Zone 3. The Zone2 valve and the Zone 3 valve were opened for three hours and then shut.The Zone 2 annulus pressure, shown as curve 125 a in FIG. 3, dropped 150psi (from approximately 840 psi to approximately 690 psi) to the levelof the tubing pressure immediately after the valve opening, and thenstarted to build up back to the formation pressure. The Zone 3 annuluspressure is shown as curve 125 b in FIG. 3. The Zone 3 pressure buildupcurve rose faster than the Zone 2 pressure buildup curve, indicatingthat Zone 3 is more permeable than Zone 2.

It will be appreciated by those skilled in the art, that immediatelyafter the opening of valves, the pressure gradient existed mainly in thedamaged zone near the well. The permeability of the damaged zone, orskin, is known to be lower than that of the undamaged formation. If thecoupling constant between the pressure gradient and the electric currentis also lower in the skin than in the formation, then the streamingpotential should increase with time initially when the pressure gradientdiffuses from the skin to the undamaged formation. At later times, thepressure builds back to the formation pressure, the pressure gradientdiminishes and diffuses deep into the formation farther away from theelectrodes, and the streaming potential decreases. The rates of theinitial increase and the subsequent decrease of the streaming potentialare determined by the permeability of the skin, the thickness of theskin, and the permeability of the undamaged formation. The streamingpotential transient recorded by electrode 118-8 in Zone 2, shownalongside with the pressure transient in FIG. 4, first rises then falls.The time scale of the fall of the streaming potential is comparable tothe time scale of the buildup of the pressure, as expected.

The Zone 2 streaming potential data recorded by all seven electrodes118-8 through 118-14 in Zone 2 are shown alongside the pressure data inFIG. 5. The Zone 3 streaming potential data are shown in FIG. 6. Thereservoir is clearly heterogeneous within each zone; individualstreaming potential curves in FIG. 5 and FIG. 6 all have very differentrise and decline rates, indicating large variations in permeability.Thus, it is seen that measuring streaming potential with an array ofelectrodes yields significantly increased information relative to theinformation that can be gleaned from a single pressure buildup curve foreach zone which would yield only the average permeability for that zone.

Careful review of the curve from electrode 118-12 in FIG. 5 reveals adouble peak. The double peak is consistent with the superimposition of afast rising and fast falling element and a slow rising and slow fallingelement. The fast element arises from flow in a fracture having a highpermeability, and the slow element arises from a flow in a formationmatrix with low permeability. This interpretation is consistent withborehole images which were obtained from a borehole imaging tool andwith modeling results discussed hereinafter.

According to the invention, the magnitude of the streaming potential isan indicator of the water fraction of flow, and it varies from electrodeto electrode. As seen in FIG. 5, there is little water production nearelectrode 118-13 (i.e., the streaming potential remains near 0 mV), andin FIG. 6, there is no or little water production near electrodes 118-16and 118-17.

The voltages of the Zone 1 electrodes are shown in FIG. 7. Since Zone 1is hydraulically isolated from Zone 2 and Zone 3 and the Zone 1 valveremained closed, the observed voltages were drifts in the electrodes.The drifts are of the order of less than one millivolt per day. Sincethe electrodes are steel centralizers exposed to the annulus fluid,drifts of such magnitude are expected.

In a later production test, Zone 2 and Zone 3 valves were shut and Zone1 valve was opened and remained open. The pressure transient and thestreaming potentials resulting from that test are shown in FIG. 8. Thelarge streaming potential measurement and double peak associated withelectrode 118-1 revealed a fracture. In addition to the fracture, thesignificant variations in streaming potential rise times (e.g., compareelectrode 118-6 with electrode 118-5, indicated large variations information permeability along Zone 1.

Turning now to FIG. 9, qualitative interpretations of the streamingpotential transient data in FIGS. 5, 6 and 8 is summarized. As seen inFIG. 9, electrode 118-1 revealed a fracture in the formation with highpermeability, while electrodes 118-2 through 118-5 and electrode 118-7indicated formation locations having medium permeability and electrode118-6 indicated a formation location of high permeability. In Zone 2,electrodes 118-8 through 118-10 and 118-14 indicated formation locationsof medium permeability, while electrodes 118-11 and 118-12 indicated aformation location or mini-zone of low permeability. In addition,electrode 118-12 revealed a fracture in the formation. Electrode 118-13indicated a formation location with no water flow. In Zone 3, electrodes118-15 and 118-18 through 118-20 indicated formation locations of highpermeability, while electrode 118-21 indicated a formation location ofmedium permeability, and electrodes 118-16 and 118-17 indicatedformation locations or mini-zone of no water flow. It will beappreciated by those skilled in the art that the streaming potentialtransient data of FIGS. 5, 6, and 8 summarized in FIG. 9 providessignificantly more detailed information than what was previouslyobtainable by pressure transient information.

The qualitative interpretations summarized in FIG. 9 are supported byforward modeling. In particular, using equation (2) for single-phaseflow, the streaming potential transients are computed from a forwardmodel of a heterogeneous formation shown graphically in FIG. 9 a. Themodeled response is shown in FIG. 9 b. As seen in FIG. 9 b, thestreaming potential recorded by an electrode placed in the highpermeability region rises faster and decays faster than that thestreaming potential recorded by an electrode placed in the lowpermeability region. Qualitatively this modeled response agrees with thedata presented in FIGS. 5 and 6. Similarly, in supporting the analysisrelated to fractures, the streaming potential transients are computedfrom a forward model of another heterogeneous formation shown in FIG. 9c. As seen in FIG. 9 d, the streaming potential transient computed froma forward model shown graphically in FIG. 9 c supports theinterpretation of the streaming potential recorded by electrode 18-12(FIG. 5); i.e., that a fracture will produce a double peaked streamingpotential transient response.

In light of the above, with the streaming potential information, it isclear that an appropriate forward modeling and inversion can be carriedout with a Laplace equation solver and a two-phase flow model (i.e.,oil/water) as discussed above with reference to equations (9)-(11) orwith a multi-phase flow model. As a result, the streaming potentialtransient data obtained can be used to quantify formation permeability,skin permeability, effective fracture permeability, horizontal andvertical permeabilities, communication between zones and wells, andreservoir boundaries in much greater detail than pressure transienttesting alone can. As a result, a better understanding of the well andthe reservoir may be obtained, leading to better management of the welland reservoir.

Turning now to FIGS. 10-19, the use of streaming potential transientinformation is shown with respect to a vertical injection well 200located in formation 205. As seen in FIG. 10, the formation 205 includesa hydrocarbon reservoir with a location identified at between 1026 ftand 1047 ft. In addition, there is a thin layer of sand at 1020.55 ft,which is hydraulically isolated from the hydrocarbon reservoir.

As seen in FIGS. 10 and 11, the well 200 includes a casing 209 aroundwhich electrical insulation 211 is provided. An electrode array 218,including electrodes 218-1 through 218-16 with associated preferablyhigh impedance voltage measurement circuitry, is mounted in or outsidethe insulation. The casing, insulation and array are cemented in placeby cement layer 217. Thus, the electrodes 218-1 through 218-16 are incontact with the cement 217 but not with the metal casing 209. As shownschematically in FIG. 11, in order to produce hydrocarbons, the casingmust be perforated with oriented perforations 219 so as not to damagethe electrodes and the connecting cables (not shown). In this case, noperforations were made above the top of the reservoir (i.e., above 1026ft.) After perforation, electrical current can leak through theperforation holes 219 to the metal casing 209. The electrical insulationof the casing is imperfect but functional, as is shown by field testresults. The bottom electrode 218-16 in FIG. 10 was used as a referenceelectrode.

With well 200, streaming potential transients were created by injectingwater into the well. The injection of water was controlled by a surfacepump 221 and a surface valve 223 (both shown schematically in FIG. 10),and monitored by a pressure sensor 225 placed between the valve 223 andthe wellhead. Initially, the injectivity of the well was too low, so thewell was acidized and fractured. A cement evaluation job showed thepossible existence of a poor cement bond. Therefore, the formationoutside the reservoir interval of interest could be fractured, andinjected water could flow into such fractures.

The uphole injection pressure is shown in FIG. 12. Before the start ofthe data shown in FIG. 12, the valve had been shut for a long time. Theinjection pressure increased suddenly at the opening of the valve, andthen periodically dropped and recovered as the pump was shut down forbrief periods of time.

The streaming potential transients sensed by the electrodes of primaryinterest inside the reservoir interval of interest are shown in FIG. 13and in an expanded time scale in FIG. 14 (it being noted that electrode218-12 failed and thus no data is shown for it). The streaming potentialtransients clearly have two components: one component changes veryquickly in response to pressure changes, and the other component changesslowly over a period of days. The fast component relates to waterflowing into fractures with high permeability. The changes in the fastcomponent in FIGS. 13 and 14 are such that the streaming potentialsdecrease with increasing injection pressure and increase with decreasinginjection pressure. This is expected since injection water carryingpositive charges moves away from the borehole and away from theelectrodes. The signs of the streaming potential of well 200 areopposite to those of the streaming potential transients shown withrespect to well 100, as the data for well 100 was collected with watercarrying positive charges moving into the borehole toward the electrodesduring production. The slow component of the transient curve comes fromwater injection from the borehole directly into the rock matrix with lowpermeability, or from the cross-flow from the fractures into the rockmatrix. The direct flow of injection water into the rock matrix isalways away from the electrodes. The cross-flow from fractures intomatrix is also away from the electrode if the electrode is situateddirectly at the fracture. The streaming potential recorded by suchelectrodes will decrease slowly as water moves into the matrix. If theelectrode is at some distance away from the fracture, the cross-flowpasses across the electrode. As a result, the streaming potential willeither decrease slowly or increase slowly as water moves into thematrix, depending on the exact location of the electrode relative to thefracture. The data in FIGS. 13 and 14 can be interpreted as showing thatelectrode 218-5 is situated directly at a strong fracture, whileelectrode 218-9 is situated a little distance away from a fracture.

The streaming potential transients sensed by the electrodes above thereservoir interval of interest are shown in FIG. 15 and in an expandedtime scale in FIG. 16. Electrode 218-2 is located very close to the thinpermeable sand adjacent a non-perforated portion of the casing. Yet, thestreaming potential of electrode 218-2 reached a value of 150 mV, whichis five times higher than the streaming potentials recorded by anyelectrode in the reservoir interval. This may be explained byunderstanding that the interval above the perforated interval had beenfractured, presumably from a cement annulus (which was confirmed by thecement evaluation job). Thus, the shapes of the streaming potentialtransients in this interval are different from those in the reservoirinterval. In this interval, the streaming potential appears to becomprised of three components: fast, medium, and slow. This is seen moreclearly with respect to electrode 218-2 in FIG. 16. A shale layerlocated between the reservoir and the thin sand layer is probablyfractured. Flow through the fractures in the shale layer has a timescale in between the flow time scales of the sand and matrix.

Turning now to FIG. 17, the streaming potentials sensed by theelectrodes. 218-13 through 218-15 below the reservoir are seen. Thevoltages sensed by these electrodes are less than 1 millivolt. Thus, itcan be concluded that there is very little injection water flowing belowthe reservoir interval.

Given the measurements made by the electrodes as shown in FIGS. 13-17, aqualitative interpretation of the streaming potential transient data maybe made and summarized as shown in FIG. 18. In particular, as seen inFIG. 18, a fracture with cross-flow exists through a shale layer atopthe reservoir; a fracture with cross-flow exists at 1028.55 feet(electrode #4); a fracture with cross-flow exists near 1037.55 feet(electrode #9); and a fracture with cross-flow exists at 1042.05 feet(electrode #11) (see FIG. 13).

The qualitative interpretation of FIG. 18 is supported by the forwardmodel shown graphically in FIG. 18 a and the modeled response shown inFIG. 18 b which show that the streaming potential from cross flows caneither have the same sign or the opposite sign to that of the fractureflow. Thus, qualitatively the modeled response successfully reproducedthe observed data of electrode 218-9 of FIG. 13 and electrode 218-2 ofFIG. 15.

The streaming potential data of electrodes 218-13 through 218-15 shownin FIG. 17 are shown in expanded time and voltage scales in FIG. 19.Before the surface valve 223 was turn on at day/time 116.43, thevoltages of electrodes 218-14 and 218-15 were stable to one digitizationlevel (i.e., to 10 microvolts). Electrode 218-13 voltage had some noisespikes up to 100 microvolts. The noise spikes happened at a very shorttime scale, were unrelated to the surface stabilities of the electrodes,and were likely due to noise picked up on the wire 235 connecting theelectrode and the surface electronics 233 (FIG. 10). These noise spikescan be lessened or eliminated by better wiring and electronics, or bydownhole electronics.

As seen in FIG. 19, the voltages of electrodes 218-13 through 218-15correlated very well with the uphole pressure data. At the opening ofthe valve 223 at 116.43, all three voltages decreased, and when the pump221 stopped momentarily near day/time 116.8, all three voltages showed asmall but visible peak. The correlation is very similar to thoseobserved in the much larger voltages measured by electrodes located inthe reservoir and in the interval atop the reservoir. Based on thisinformation, it can be concluded that the electrode stability for thecemented electrodes is of the order of 10 micro-volts and signal levelsof 100 micro-volts are adequate to determine reservoir properties ofinterest. The stability of the cemented electrode array 218 is at leastone hundred times better than the exposed centralizer electrodes 118shown with reference to well 100.

According to another aspect of the invention, the electrodes of theelectrode array utilized to sense and measure streaming potentialtransients are preferably covered or coated with a semi-porous coveringmaterial (such as cement), whether utilized as centralizers as shownwith reference to a sand-screen completion or in other permanentinstallations, or when used in MWD or wireline applications as discussedhereinafter. The semi-porous covering material should have a significantelectrical conductivity but a very low permeability so that ions canreach the electrode to enable voltage measurements, but no new fluidreaches the electrode surface during the time period of measurement. Thesurfaces of the electrodes are in a stable chemical environment, whichgives rise to measurement stability. A presently preferred semi-porousmaterial is cement, although a semi-porous ceramic, clay, or othermaterial could be utilized. As an alternative, liquid junctionelectrodes can be utilized, as the semi-porous plug of a liquid junctionelectrode stops fluid movement but allows ionic diffusion. A stableelectrode allows the measurement of a transient over a longer period oftime, thereby permitting an analysis deeper into the formation, and alsopermitting measurements at weaker signal levels.

With the streaming potential measurements described with reference toFIGS. 10-19, it will be appreciated that determinations can be made ofthe formation (matrix) permeabilities and the effective fracturepermeabilities along the well utilizing equations (9) through (11) asdiscussed above and by considering fractures as a thin medium with givenpermeability. In addition, the streaming potential measurements can beutilized to obtain real time monitoring of fracturing jobs. For example,when well 200 was fractured, the target was the middle reservoirinterval of interest, and the fracturing of the upper interval was notdesired. However, the injected water did not go where it was desired.Had the streaming potential data been acquired during the fracturingprocedure, it would have been observed at a very early time that thefracturing fluid was moving mainly toward the upper interval (above thereservoir). The fracture job could then have been stopped, a cementsqueeze job applied, and the fracture plan properly executed.

Turning now to FIG. 20, a formation 305 is seen traversed by an openhole completed well 300 having a tubing 306 extending therein. Aninsulated sonde 311 is shown around the tubing with electrodes 318-1,318-2 . . . disposed on the insulated sonde 311. Thus, the tubing 306 isessentially just a conveyance means for moving the sonde 311 to desiredlocations. Other conveyance means, which are preferably relativelysolid, but somewhat flexible, could be utilized. Those skilled in theart will appreciate that wires connecting the electrodes, measuringelectronics, and telemetry or data storage, which are standard in theart, are provided in, on, or with the sonde 311 and electrodes 318 butare not shown in FIG. 20.

A cased hole completion is shown in FIG. 21, with a formation 405traversed by a well 400. The well includes an insulated tubing 406, anda casing having conductive electrode portions 418-1, 418-2, 418-3, . . .separated by electrically insulated portions 416 which are cemented intothe well by cement 417. Thus, the metal casing serves as an electrodearray with individual sections of the casing electrically isolated fromone another. The casing sections may be regular casing sectionsconnected by isolation joints, or specially designed casing sectionsmade of two or more electrically isolated subsections. As seen in FIG.21, the electrodes 418 are in contact with the cement 417 and with thefluid inside the casing. If the tubing 406 inside the well is metallic,the tubing is preferably electrically insulated or partially insulated.

According to another embodiment of the invention, a tool and method formeasuring streaming potentials while drilling a borehole is provided. Inparticular, during drilling, a pressure difference between the formationand the borehole creates mud invasion and pressure transients, and thus,streaming potential transients. In wells drilled with an oil-based mud,a streaming potential will exist if the mud contains a water fraction.

Turning now to FIG. 22, a schematic design of an while-drillingstreaming potential tool 510 is shown in borehole 500 surrounded byformation 505. Drilling tool 510 includes a drill bit 507 and electrodes518-1, 518-2, 518-3 . . . , 518-R (all preferably coated with asemi-porous covering such as cement) mounted on electrically insulatedsections 511-1, 511-2, 511-3 of the drill pipe 515. The electrodes 518move with the tool 510. Thus, different electrodes in the array willsense at different points in time the streaming potential transient at afixed spatial point. The spacing between the electrodes 518 in the arrayand the drilling speed determines the temporal sampling rate of thestreaming potential transient. In other words, the time at whichelectrode 518-2 is located at a particular previously measured byelectrode 518-1 is dependent upon both the drilling speed and thedistance between the electrodes. In the embodiment of FIG. 22, the topelectrode 518-R is used as the voltage reference electrode, as it isfarthest from the drill bit and will often arrive at locations in theformation when the streaming potential transient has already reachedsteady state values. Those skilled in the art will appreciate that wiresconnecting the electrodes, measuring electronics, and telemetry, whichare standard in the art, are provided in, on, or with the LWD tool 510but are not shown in FIG. 22. A processor 550 and associated datastorage 560 are shown which are used to obtain answer products are shownin FIG. 22. It will be appreciated that the processor 550 and datastorage 560 are applicable to the other embodiments as well, althoughthe processor may utilize different forward and inverse models.

With the while-drilling tool 510, the streaming potential measurementsmade are passive voltage measurements, which can be made in a highlyresistive borehole by using high impedance electronics. In wells drilledwith oil-based mud, the electrodes need to be as large as possible andplaced as close as possible to the formation to reduce electrodeimpedance.

It will be appreciated by those skilled in the art that in order toproperly analyze the data obtained by the LWD tool 510, a model ofmudcake built up during drilling should be included in the forwardmodel. Accurate models such as disclosed in E. J. Fordham and H. K. J.Ladva, “Crossflow Filtration of Bentonite Suspensions”, Physico-ChemicalHydrodynamics, 11(4), 411-439 (1989) can be utilized.

Given the while-drilling tool 510 and an appropriate model, thestreaming potential information obtained by the tool and processed canyield various answer products. Since the streaming potential transientscreated by drilling will change rapidly with time for a formation withhigh permeability and slowly for formation with low permeability, withan inversion model that contains the mudcake built-up model, formationpermeability of the invaded zone and the uninvaded zone can be obtained.

With the LWD tool 510 and an appropriate model, a system for earlydetection of drilling fluid loss may be implemented. In particular,there may be sudden fluid loss from natural or induced fractures duringdrilling. In that case, streaming potential will rise instantaneously asfluids rush into the formation. The changes in borehole pressure will besomewhat slower, since the borehole has a storage capacity. Noticeablefluid loss at the surface will happen much later. For drilling inducedfractures, large changes in the streaming potential will be detectablelong before the fractures becomes serious. Therefore, monitoring of thestreaming potential measurements can be used for early detection offluid loss.

Likewise, the streaming potential information can be utilized for theearly detection of abnormal formation pressures. For example, if theformation pressure becomes higher than the borehole pressure, the signsof the streaming potential will reverse. This reversal of sign will beobservable before sufficient amount of fluid has flowed into theborehole for the pressure kick to be observable. The build-up of theflow reversal may happen over a short but finite period of time as theabnormal pressure zone is being drilled. Any reversal of flow will beimmediately observable in the streaming potential measurements.Therefore, streaming potential measurements have value in the earlydetection of abnormal formation pressure.

Turning now to FIG. 23, another embodiment of the invention is seen. InFIG. 23, a wireline streaming potential tool 610 is provided. Thewireline tool 610 is shown suspended by a cable 611 in a borehole 600(having mud cake 607) traversing a formation 605. The wireline tool 610is provided with an insulated sonde 616 on which an array of electrodes618-1, 618-2, 618-3 . . . including a reference electrode 618-R, andassociated preferably high impedance voltage measuring circuits areprovided. The electrodes are preferably coated with a semi-porousmaterial such as cement. In addition, tool 610 includes one or morepreferably retractable arms 631 on which one or more cutting edges 635are mounted. The cutting edges 635 are designed to cut slits across themudcake 607 as the wireline tool is moved through the borehole. Thecutting edges may be made with a polycrystalline diamond compound (PDC).Because there is a large overbalancing pressure difference between theformation and the borehole (most of the pressure difference existsacross the mudcake), after the cutting edges 635 slit the mudcake 607, anew mudcake will quickly build up in the slit to stop the fluid flow. Inthe mean time, a pressure transient has been created in the formation605. In wells drilled with oil-based mud, streaming potential transientswill be created if the mud has a water fraction.

As will be appreciated by those skilled in the art, the electrodes 618move with the tool 610 in a continuous logging mode. Differentelectrodes in the array sense the streaming potential transient at afixed spatial point. The spacing between the electrodes in the array andthe logging speed determines the temporal sampling rate of the streamingpotential transient. The top electrode 618-R is used as the voltagereference electrode, as it is farthest from the cutting edges and nostreaming potential transient has yet been created there. Wiresconnecting the electrodes, measuring electronics, and telemetry areprovided but not shown in FIG. 23.

As previously mentioned, the arms 631 are preferably retractable. As aresult, the cutting edges 635 can be retracted where streaming potentialinformation is not desired, and the tool used for repeated runs toacquire streaming potential data over long period of time, if desired. Agamma ray detector 640 is provided in order to help align data fromrepeat runs.

As was discussed above with reference to the LWD tool 510, streamingpotential measurements are passive voltage measurements which can bemade in a highly resistive borehole by using high impedance electronics.In wells drilled with oil-based mud (without a water fraction), theelectrodes are preferably relatively large (by way of example and notlimitation, twelve inches by two inches) and are preferably placed onarticulated pads (not shown) or on a skid sonde to insure close contactwith the formation.

Using the wireline tool 610, the spurt loss from the cutting of mudcakeis likely to happen over a short time period compared with the timeneeded for the pressure transient to diffuse beyond the invaded zone. Ifthat is the case, then the source of the streaming potential transientcreated by the cutting of mudcake can be treated as a delta function oftime. The inversion of the data for a short period of time can becarried out without any input from the mudcake build-up model. After thespurt loss, the mudcake will build back up by a static process. Thethickness of the mudcake will increase with the square root of time. Theinversion of streaming potential data over a longer period of time witha mudcake that increases with the square root of time is still quiterobust.

The mud invasion is a continual process even with a good mud system. Thestreaming potential transients created by the mud invasion are likely tobe measurable when the logging time is not too far away from the timewhen the well is drilled or reamed. Thus, the tool shown in FIG. 23 withthe cutting edges retracted (or without the arms and cutting edges) canrecord the streaming potential created by the previous drilling and/orreaming, and the continual mud invasion. In such a situation, a modelfor a long measuring period and a mudcake build-up will be utilized forinterpreting the streaming potential data collected. Thus, it will beappreciated that the wireline streaming potential tool can be used withappropriate modeling and inversion to provide measurements of formationpermeability in the invaded zone, beyond the invaded zone, and in thefar zone, continuously along the borehole. The transients acquired overlong periods of time without the cutting blade will help to determinethe permeability in the far zone.

The ability of the wireline tool of FIG. 23 to detect streamingpotential transients and provide qualitative determinations is supportedby the forward model of FIG. 23 a and the results of the model shown inFIGS. 23 b-23 e. It is assumed in the model of FIG. 23 a, that the spurtloss from the cutting of mudcake happened over a short time periodcompared with the time needed for the pressure transient to diffusebeyond the invaded zone. The source of the streaming potential transientcreated by the cutting of mudcake was treated as a delta function oftime. After the spurt loss, it was assumed that a new mudcake stoppedall further flow.

FIG. 23 b shows that the early time transients are insensitive to theuninvaded zone. It takes a time interval given by equation (4) for thepressure transient to diffuse to the uninvaded zone. FIG. 23 c showsthat both early time streaming potential and late time streamingpotential are sensitive to the permeabilities of the invaded zone. Thefact that early time data and late time data are sensitive topermeabilities of different zones makes the inversion algorithm quiterobust.

FIG. 23 d shows the dependence of the streaming potential on thethickness of the invaded zone. Equation (4) shows that the time it takesfor the pressure transient to diffuse through the invaded zone dependson the invaded zone thickness Δ and invaded zone permeability k throughthe combination Δ²/k. Equation (8) shows that in approaching the steadystate, the streaming potential from the invaded zone depends on Δ and kthrough the combination Δ/k. The difference between these twocombinations suggests that the thickness and the permeability of theinvaded zone can be individually determined by inversion.

The results of inversion with synthetic data calculated from the forwardmodel and 5% added noise are shown in FIG. 23 e. The inverted values ofthe invaded zone permeability, uninvaded zone permeability, and thethickness of the invaded zone all agree very well with the input valuesused in the forward model.

There have been described and illustrated herein several embodiments ofapparatus and methods for measuring streaming potentials andcharacterizing earth formation characteristics therefrom. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular tools andelectrode arrangements have been disclosed, it will be appreciated thatmodifications can be made, provided the tool or arrangement includes anelectrode array capable of measuring streaming potentials. Thus, forexample, the invention could be modified so that a two-dimensional arrayof electrodes can be utilized in certain circumstances in order toprovide azimuthal streaming potential information. It will therefore beappreciated by those skilled in the art that yet other modificationscould be made to the provided invention without deviating from itsspirit and scope as claimed.

1. A method for investigating an earth formation traversed by a holecomprising: a) using at least one sensor in or about the hole to sensetransient streaming potential DC voltages over a period of time; and b)identifying a fracture in the earth formation using said streamingpotential DC voltages or indications thereof.
 2. A method according toclaim 1, further comprising: c) inducing generation of said transientstreaming potential DC voltages.
 3. A method according to claim 2,wherein: said inducing comprises causing a change in relative pressurebetween the hole and the formation.
 4. A method according to claim 2,wherein: said inducing comprises cutting mudcake on said hole.
 5. Amethod according to claim 2, wherein: said inducing comprises injectingliquid into the hole.
 6. A method according to claim 5, wherein: saidliquid comprises water.
 7. A method according to claim 5, wherein: saidliquid comprises water contained in drilling mud.
 8. A method accordingto claim 2, wherein: said inducing comprises injection of at least oneof completion fluid, cement, gravel, acids, fracturing, and propellantinto the hole.
 9. A method according to claim 2, wherein: said inducingcomprises changing a rate of production of formation fluids through thehole.
 10. A method according to claim 1, wherein: said identifying afracture comprises identifying a streaming potential voltage transienthaving a double peak profile.
 11. A method according to claim 1,wherein: said using at least one sensor comprises using a plurality ofsensors.
 12. A method according to claim 10, wherein: said plurality ofsensors are at least partially electrically insulated from each other.13. A method according to claim 11, wherein: said using a plurality ofsensors comprises moving a wireline tool with said sensors through saidhole.
 14. A method according to claim 13, wherein: said wireline toolincludes means for cutting mudcake about the hole.
 15. A methodaccording to claim 11, wherein: said using a plurality of sensorscomprises rotating a tool having a drill bit and said sensors.
 16. Amethod according to claim 11, wherein: said hole is an open holecompletion, and said using a plurality of sensors comprises using aplurality of sensors deployed on an insulating sonde placed in said openhole.
 17. A method according to claim 11, wherein: said using aplurality of sensors comprises locating said sensors on centralizers ina sand-screen completions.
 18. A method according to claim 11, wherein:said using a plurality of sensors comprises locating said sensors onelectrically isolated casing sections in a cased-hole completion.
 19. Amethod according to claim 11, wherein: said using a plurality of sensorscomprises locating said sensors in insulation surrounding a casing in acemented completion.
 20. A method for investigating an earth formationtraversed by a hole comprising: a) locating a plurality of sensors in orabout the hole; b) inducing generation of transient streaming potentialDC voltages in the formation; c) using the sensors to sense saidtransient streaming potential DC voltages over a period of time; and d)identifying a fracture in the earth formation by identifying a doublepeak profile in said streaming potential DC voltages or indicationsthereof.
 21. A method for investigating an earth formation traversed bya hole, comprising: a) using at least one sensor in or about the hole tosense transient streaming potential DC voltages over a period of time;and b) processing said transient streaming potential DC voltages orindications thereof to accomplish at least one of monitoring a fracturejob, evaluating a fracture job, monitoring cementing, monitoring gravelpacking, monitoring acid flow, and evaluating an acidization job.
 22. Amethod according to claim 21, wherein: said using at least one sensorcomprises using a plurality of sensors.